1. Field of the Invention
Some embodiments of this invention relate to the field of precise location of concealed objects and linear utility conveyances, and, in particular, to precise positioning using a distributed sensor network.
2. Discussion of Related Art
Precise positioning, locating, and tracking of underground infrastructure is difficult under many circumstances, especially in dense urban environments, where buried or obscured pipes and other utilities are co-located in conduits within constrained rights-of-way. In those areas, active dipole transmitters, sometimes known as ‘sondes,’ may be used to track the path of an underground conduit, or the path of an underground directional drilling tool during the placement of new utility lines. Similarly, active signals are placed on linear utility conveyances (cables and pipes), to facilitate precise location of specific lines from a signal often distorted with similar signals from non-targeted lines.
Conventional precise location systems use a variety of methods to compute the position of a line or sonde-transmitter. Most often, the line is energized by a transmitter at a point away from the locate region of interest, where the line is accessible. In the case of a sonde, the device itself is a battery-operated active transmitter and is placed, towed, drilled, or pushed to the locate region of interest. A precise location receiver monitors the signal transmitted by the transmitter and derives an estimate of the offset, depth, and range to the targeted line or sonde.
Some precise location systems, known as real-time locating systems (RTLS) use ultra-wideband (UWB) technology if the entire system is aboveground and not significantly impacted by metal obstructions in the signal path. For UWB locating systems, short wavelength radio frequency (RF) pulses in the GHz range are used to measure time delay estimates from a transmitter source to each receiver, from which distance is determined using the known speed of propagation, i.e., the speed of light. Alternatively, path loss measurements from which distance can be inferred through a known exponential reduction in the omni-directional electric-field signal strength with distance, as long as the transmitter power is known. Lower frequency ranges are used for signal strength measurements, but poor accuracy results for situations when there are obstructions in the signal path between transmitter and receiver. WiFi-based RTLS are a typical example of signal strength-based aboveground locating systems that employ multiple distributed receivers. In either type of RTLS system, these measurements are used as input to a multi-lateration positioning algorithm to compute the location of the transmitter(s).
For underground and underwater precise location problems, the RF transmitter frequency must be constrained to less than 100 kHz to avoid high path loss. Since the detection range is from anywhere between a meter to a few tens of meters, sensors that detect magnetic fields are preferred, since at low frequencies the magnetic field can be closely controlled at the point of transmission by maintaining a fixed current flow through an underground linear conveyance, such as a cable or pipeline, or a point source, such as a dipole antenna. With a fixed current, the emitted AC magnetic field strength is stable and can be characterized by physical models. Point sources, like sonde transmitters, follow a dipole field model with 1/r3 decay with distance, while linear conveyances follow a cylindrical model with 1/r field strength decay with distance.
Therefore, there is a need for better, more precise, locating equipment.